ieee80211: definitions for reduced neighbor reports

Add the necessary definitions to parse reduced neighbor
report elements.

Signed-off-by: Tova Mussai <tova.mussai@intel.com>
[change struct name, remove IEEE80211_MIN_AP_NEIGHBOR_INFO_SIZE]
Link: https://lore.kernel.org/r/20200528213443.4f9154461c06.I518d9898ad982f838112ea9ca14a20d6bbb16394@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>